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RE: MORE ABOUT TRUSSES...
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- Subject: RE: MORE ABOUT TRUSSES...
- From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
- Date: Wed, 30 May 2001 12:23:28 -0400
There are "Rules of Thumb" for depth to span ratios for trusses that have been shown to produce "economical" trusses. Basically, all of them relate to the moment diagram for a uniformly loaded member. Usage might require deeper or shallower depths. (Note joist depth/span ratio.) These are from memory right now as I haven't dragged out my references: Pitched: depth = 1/8 span Parallel: depth = 1/10 span Bowstring: depth = 1/8 span Continuous: depth = 1/12 to 1/15 span Joists usually have a depth of 1/25 to 1/27 span. Of these, the relationship to the moment diagram can be seen most clearly with the bowstring truss. The force in the top chord and the force in the bottom chord remains essentially constant from the middle of the span to the end of the span resulting in uniform sized chord members. Web members carry very little force. Here in the desert southwest, roof mounted air condidioners and coolers are common and allowances should be made for something that would be mounted on the roof. The large (60" X 60") evaporative coolers, with soaked pads and water in the reservoir, will weigh upwards of 1,100 pounds. HTH A. Roger Turk, P.E.(Structural) Tucson, Arizona Greg Effland wrote: ----------------Begin quoted message-------------- First I would like to more correctly state something I had recommended to you about stitch welding the double angles together... I think someone else pointed this out correctly but when welded together the kl/r between welds of each member (angle) should not exceed 3/4 of the kl/r of the composite member (as per AISC ASD 9th Ed. Section E4 Paragraph 5) I think applying the 5 kips (if desired) should be done at bottom panel points. I think the check with the loads at the top panels might be overly conservative. If it were to be used as a hoisting point most people would go to the closest point... besides the roof panel may not allow someone to go above the top chord. Collateral Load is essentially Dead Load. It should be included with dead load in any "Gravity" load combinations and NOT included with wind uplift load combinations. The theory behind this approach is that the "collateral" load may not be attached to every supporting member, it might be connected to every other one or something like that. If assumed it may not connect to each supporting member then it is conservatively placed with gravity combinations and conservatively NOT placed with uplift combinations. Quick Moment/Depth (M/D) check... This can be done at any length along the truss. Typically I would start at the highest moment. Since most trusses are simple span the highest moment occurs in the middle of the span. For open web frames (I think this is the case you are asking about) there might be a fixed connection at the sidewall columns. This will alter the moment diagram and this should be taken into account. What I would do , and keep in mind this is not exact and only provides a starting point, is calculate the maximum moment and divide this number by the assumed total depth of the truss at that point. Simplified Example: 20 ft trib width 40 psf roof load 40*20=800 plf Max. Moment (Assume 110 ft simple span) = (wl^2)/8 = 14,520 kip-in Assumed depth = 4 ft = 48 inches M/d = 302.5 kips Assuming 0.6*Fy*Area (Fy=50 ksi, High Strength Steel) *** Might consider using 0.66*Fy or 0.75*Fy to make sure you don't miss a lighter case that may work. 0.6*Fy= 30 ksi Area(Required)= (M/d)/(0.6*Fy) = 10.08 Inches Use a chord with Area >= 10.08 inches (Usually pick a selection that is readily available, refer to Modern Steel Construction, they annually publish charts showing which sections are more available then others... or consult your standard steel supplier) This gives you a ROUGH area to start with. A chord with the area less than calculated above would likely not work. If .66Fy was used then the possibilty of a section with less area working decreases. If used .75Fy was used then I would think only the rarest cases (if any) would work with less area than calculated above. Hope this helps, Greg Effland, P.E. -----Original Message----- From: Juan José Treff De la Mora [mailto:jjtreff(--nospam--at)hotmail.com] Sent: Tuesday, May 29, 2001 5:47 PM To: seaint(--nospam--at)seaint.org Subject: MORE ABOUT TRUSSES... Hi everyone, Recently I posted a question on roof trusses. Many people responded me with very good advise but I have some new doubts and would like to know your opinion. Mark advised me to include 5 extra kips at the worst joints or nodes due to lifting. Did I understand right? How would you pick the "worst ponits"? Do you do it only in the lower chord nodes? Then John advised me to add 10 psf for ceilling weight. And Greg told me to add from 3 to 10 psf for collateral loads. Do I add this loads to the Dead Load condition, to the Live Load condition or as a new Collateral load condition? For the initial truss depth, Greg adviced me to do a quick moment/depth calc and check against ASD tension check. Coul you be a little bit more explicit? Thank you all for your kind and valuable help. Regards, Juan José -------------------End quoted message---------------- * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org
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